This section provides background information related to the present disclosure which is not necessarily prior art.
Technical Field
The method relates to a method and a device for the slag bath gasification of solid fuels in the slag bath gasifier with increased performance, a wider range of application for solid fuels and improved gas quality. The method and device according to the invention allow the gasification of coals with higher fine-grained proportions and/or additionally of fine-grained and dust-type fuels.
Discussion
The gasification of coarse-grained solid fuels, i.e. of coarse-grained coal and/or carbonaceous solids with grain sizes higher than approximately 6 mm and smaller than approximately 100 mm preferably takes place in a fixed bed according to the fixed bed high-pressure gasification method FDV or to the slag bath gasification SBV. The latter ist also known under the designation British Gas Lurgi BGL gasification. The fuels are supplied together with flow media by pressure locks overhead into the slag bath gasifier. The following zones from top to bottom are ideally constituted in the fixed bed (in fact a moving bed) that extends over the height of the slag bath gasifier: drying zone, pyrolysis zone, gasification zone, oxidation zone (blow mould) and slag bath. The slags are removed as liquid slags at the bottom of the gasifier. The gasifying agents are blown in over gasifying agent nozzles that extend into the gasifier and that are directed onto the slag bath. The raw gas vent for the raw gas that has been formed is situated on the head of the fixed bed gasifier.
The gasifying agents essentially consist of technical oxygen and vapour. The ratio between vapour and oxygen ratio (vapour/oxygen ratio) DSV, preferably indicated in the units kg vapour/m3 (S.I.), oxygen (100% vol.), ranges between 0.8 and 1.6 kg/m3 (I.S.) depending on the ash melting temperatures. Under approx. 0.8 kg/m3 (I.S.), thermal load of the gasifying agent nozzles is too high and above approx. 1.6 kg/m3 (I.S.) the ashes are no longer correctly melted.
The slag bath gasification has two general disadvantages. The first one consists in that the percentage of fine grain <6 mm to values of <approx. 5 to <approx. 20% mass percent is very limited; for non-agglomerating coal to values <approx. 5% mass percent and for agglomerating coal to values <approx. 20% mass percent. Otherwise, it comes to impairments of the good flowing of the filling and to undesirable sweeping away and discharge of dust or even to coarse-grained solids with raw gas out of the gasifier. The second disadvantage relates to the low temperatures of the dust containing raw gas flowing out of the gasifier that range between approx. 400 and 900° C. depending on the type of the fuels used. The raw gas temperatures are too low for allowing the tars formed in the pyrolysis zone and the heavy hydrocarbons to decompose. This determines on the one hand a costly gas cleaning and, on the other hand, a reduction of the synthesis gas yield of CO and H2 related to the fuels used.
In order to compensate this disadvantage, it has been proposed in DE 10 2007 006 979 B4 to inject gasifying agents for the post-gasification into the free space above the fill of the fixed bed. The temperature of the raw gas should thus be increased to at least 800° C. in order to crack the heavy hydrocarbons and to convert them into synthesis gas. This solution could not be successfully achieved until now since the flame spread upwards is too big and it is to be feared that non reacting oxygen flows out of the gasifier with the raw gas. Up to now, it has not been possible to find technically more favourable solutions for the recovery of fine fuels and for improving the gas yield. A further use (generally combustion) of the coal fines has to be assured instead of gasification. However, a bigger part of the coal fines cannot be used economically and has to be disposed of as heaps.